Assortment of geometry instruments; a protractor (centre, used for measuring angles), set squares (providing fixed angles) and a ruler. When photographed through cross polarised light, coloured patterns are seen on the plastic instruments. These indicate stress patterns, with areas of similar colour representing regions under similar degrees of stress. This phenomenon, known as photoelasticity, is applied in engineering design where a transparent plastic model with loads proportional to a real object may be made to determine stress conditions in the real object.